Abstract: The magnetic object’s location and shape could be estimated more precisely with its vector information than scalar information. Therefore, the conversion from scalar information to vector information should be studied. With prescribed direction of total field, the rigor formula could be deduced based on the relationship between total field magnetic anomaly and magnetic anomaly components in frequency domain. The method has been testified by sphere and cuboids’ simulation model. The conclusion shows that the two calculated horizontal components match its theoretical value perfectly, and the vertical component has only little error; the precision is almost the same no matter how section survey line traverses the projection point, moreover, the precision decreases slightly if the geomagnetism direction has little error.